Polymerization of unsaturated monomers with radiation in the presence of salts

ABSTRACT

Improved process for the preparation of water-soluble, substantially linear, high molecular weight polymers, comprising irradiating an aqueous solution of an ethylenically unsaturated monomer and a water-soluble salt under controlled conditions of concentration, radiation intensity, conversion, and total radiation dose. The polymers may be obtained in aqueous gel form or recovered in the form of powder. The polymers are useful as flocculating, thickening, and mobility control agents.

This invention relates to a process for the preparation of water-soluble, substantially linear, high molecular weight polymers. More particularly, this invention relates to an improved process for obtaining water-soluble, substantially linear, high molecular weight polymers of ethylenically unsaturated monomers which comprises irradiating an aqueous solution of ethylenically unsaturated monomer and salt.

The expression "water-soluble polymer" when used in this application means that at least 90% by weight of the polymer dissolves in water in accordance with the following test: the polymer is dispersed in water, in a weight ratio of 1 to 1000 respectively, by stirring the mixture with a magnetic stirrer at 200 r.p.m. for a period of two hours. A 100 gram aliquot of the resultant dispersion-solution is filtered through a 200 mesh screen. The 200 mesh screen is then washed with 300 cc. of distilled water. The filter cake remaining on the 200 mesh screen is dried at a temperature of about 115° C. until constant weight. The dried portion, which is the insoluble part of the polymer, must be less than 10 weight percent of the polymer contained in the 100 gram aliquot. This is determined by dividing the weight of the dried portion by the weight of polymer in the 100 gram aliquot. The water temperature used for this test is 25° C.

The expression "cationic polymer" when used in this application means polymer consisting of 1% to 100% of polymerized cationic monomer and 0 to 99% of polymerized nonionic monomer.

The expression "anionic polymer" when used in this application means polymer consisting of 1% to 100% of polyermized anionic monomer and 0 to 99% of polymerized nonionic monomer.

The expression "ampholytic polymer" when used in this application means polymer containing at least 1% by weight of polymerized ionic monomer and containing both polymerized anionic monomer and polymerized cationic monomer.

The expression "nonionic polymer" when used in this application means polymer containing from 0 to less than 1% by weight of polymerized ionic monomer.

The prior art reveals a number of methods for preparing water-soluble polymers of ethylenically unsaturated monomers. For example, ethylenically unsaturated monomers have been polymerized in suspension, in emulsion, and in solution employing chemical, free-radical initiators to initiate polymerization. In suspension or emulsion polymerization, the chemical catalyst is added to a mixture of organic solvent, water, monomer, suspending or emulsifying agents, and, optionally, a polymer precipitant. By these techniques, it is possible to obtain high conversion of monomer to polymer in products of high molecular weight. However, the polymer must be separated from excess quantities of solvent and additives in order to obtain the product in useful form. Solution polymerization via chemical catalyst has not been satisfactory for forming high molecular weight polymers because the polymerization cannot be carried out to high conversion and high molecular weights at practical concentrations of monomer in solution. Moreover, chemically catalyzed polymerizations require careful control of polymerization temperature to avoid decreases in molecular weight or premature cross-linking and insolubilization of the polymer. In order to achieve practical levels of conversion, molecular weight, and water-solubility, the rate of such polymerizations must generally be controlled to require reaction periods of several hours, usually from several hours to several days. It has been proposed to increase the polymerization rate of chemically catalyzed polymerization reactions by conducting the polymerization in the presence of a water-soluble salt. While the salt does appear to increase the polymerization rate without substantially reducing the molecular weight of the polymer, the reaction time required for the preparation of very high molecular weight polymers is still unduly long.

In recent years, attention has been focused on the polymerization of ethylenically unsaturated monomers under the influence of high energy ionizing radiation, for example, gamma radiation. Numerous processes have been developed for the preparation of water-soluble, substantially linear, ultra-high molecular weight polymers of ethylenically unsaturated monomers. The reaction time required for the preparation of high molecular weight polymers under the influence of gamma radiation is very short, usually less than one hour. One disadvantage of these radiation-induced polymerization processes, however, has been that at the higher monomer concentrations and lower radiation intensity required to produce the high molecular weight polymers, the polymerization process must be terminated before all the monomer has been converted to polymer in order to avoid undesirable amounts of branching and cross-linking of the polymer chains which results in the formation of water-insoluble polymers. This branching and cross-linking of the polymer to form water-insoluble products occurs because the number of free radicals formed directly or indirectly by irradiation of the polymer molecules increases as the polymer concentration in the irradiated solution increases, and therefore, the probability of two of these polymer radicals finding each other in close proximity to react and form linkages between the polymer molecules is greater, and because the rate of polymerization decreases as the monomer to polymer conversion increases, and therefore the radiation dose required per unit of percent conversion increases. In most cases the radiation required to convert the last few percentage points of monomer to polymer is a significant portion of the total radiation dose. Such a big radiation dose delivered to the polymerization system at a time when the polymer concentration is high will cross-link the polymer and result in the formation of water-insoluble products. Therefore, if radiation is used for the synthesis of very high molecular weight, water-soluble polymers from concentrated aqueous solutions, the polymerization reaction must be stopped short of complete conversion if cross-linking and insolubilization of the polymer is to be avoided.

In accordance with the present invention it has been found that very high molecular weight, substantially linear, water-soluble polymers may be prepared in very short reaction time and at conversion of monomer to polymer levels of up to substantially 100% by a process which comprises irradiating an aqueous monomer solution containing at least one water-soluble salt of the class hereinafter defined, under carefully controlled conditions of monomer concentration, radiation intensity, total radiation dose, and monomer to polymer conversion as set forth hereinafter. The presence of the salt in the aqueous solution increases the rate of polymerization, and thereby shortens reaction times. More importantly, and unexpectedly, the salt increases significantly the molecular weight of the polymer formed at any given level of conversion of monomer to polymer. The salt also gives a more linear polymer, as shown by its low Huggins value, at any given molecular weight. The presence of the salt also permits the formation of ultra high molecular weight polymers wherein substantially all of the monomer is converted to polymer.

The water-soluble salts which may be used in the process of this invention are those water-soluble salts which are sufficiently soluble in the aqueous reaction medium used herein to furnish a solution containing at least 3% by weight of dissolved salt, based on the total weight of solution, and which are incapable of forming free radicals scavengers under the influence of high energy ionizing radiation. In this latter category nitrate and nitrite salts are specifically excluded. Examples of salts which are useful in the process of this invention are: potassium sulfate, potassium chloride, potassium fluoride, potassium bisulfate, tribasic potassium phosphate, dibasic potassium phosphate, monobasic potassium phosphate, sodium sulfate, sodium bisulfate, sodium chloride, tribasic sodium phosphate, dibasic sodium phosphate, monobasic sodium phosphate, lithium sulfate, lithium bisulfate, lithium chloride, ammonium sulfate, ammonium bisulfate, ammonium chloride, tribasic ammonium phosphate, dibasic ammonium phosphate, monobasic ammonium phosphate, ammonium fluoride, aluminum sulfate, and aluminum chloride. A preferred class of salts includes sodium chloride, sodium sulfate, sodium bisulfate, potassium chloride, potassium sulfate, potassium bisulfate, ammonium chloride, ammonium sulfate, and ammonium bisulfate. The sodium, potassium, lithium, and ammonium salts may be used to polymerize any of the ethylenically unsaturated monomers described hereinafter; whereas the aluminum salts may be used only to polymerize monomer compositions consisting of from 5% to 100% of cationic monomer and from 0% to 95% of nonionic monomer.

Mixtures of more than one salt may also be used. It has been unexpectedly discovered that in certain instances, a combination of salts gives a high molecular weight polymer and a faster reaction rate than can be obtained with a single salt. Preferred combinations of the above salts include mixtures of more than one of the following salts: potassium chloride, potassium sulfate, potassium dibasic phosphate, potassium monobasic phosphate, sodium chloride, sodium sulfate, sodium dibasic phosphate, and sodium monobasic phosphate, wherein the salts are selected to provide at least 10% by weight of one or more salts having a common anion and at least 10% by weight of one or more salts having a second common anion which is different from the first said common anion. Particularly preferred combinations of salts include mixtures of two or more salts selected from the group consisting of sodium chloride, sodium sulfate, and potassium dibasic phosphate.

The amount of salt used will depend on the particular salt and reaction conditions used and on the molecular weight and reaction rate desired. In general, the amount of salt employed is from 3%, based on the total weight of solution, to the saturation point of the salt in the monomer solution at 35° C., and preferably from about 4% to saturation. A particularly preferred amount of salt is from about 6% to saturation. Lower amounts of salt are less effective and, for practical purposes, produce no increased benefits. Larger amounts of salt may be used, but do not increase benefits. It has been found that provided all other variables are kept constant, the molecular weight (intrinsic viscosity) and linearity of the polymer product increase as the amount of dissolved salt increases. A preferred concentration for sodium chloride, potassium chloride, and ammonium chloride is from 8% to saturation. A preferred concentration for sodium sulfate, potassium sulfate, and ammonium sulfate is from 4% to saturation.

The radiation-induced polymerization is effected, in accordance with this invention, by subjecting an aqueous solution of salt and at least one ethylenically unsaturated monomer to high energy ionizing radiation. The radiation employed may be particulate or electromagnetic in nature, and includes accelerated electrons, protons, neutrons, etc., as well as X-rays and gamma rays.

The polymers of the invention may be prepared from water-soluble monomers conforming to the formula ##STR1## mixtures of such monomers, or water-soluble mixtures of at least one of such monomers with up to 50% by weight of an ethylenically unsaturated monomer selected from the group consisting of vinyl sulfonic acid, alkali metal salts of vinyl sulfonic acid, diacetone acrylamide, and mixtures thereof. In the above formula, R represents hydrogen or methyl and Y represents --NH₂, --OM, ##STR2## wherein M is hydrogen, H₄ N⁺, alkali metal, or any other cation yielding a water-soluble, polymerizable compound, R₁, R₂, and R₃ are 1 to 4 carbon alkyl radicals and X is an anion. Illustrative examples of monomers conforming to the formula include acrylamide, methacrylamide, 2-acrylamido-2-methylpropanesulfonic acid, acrylic and methacrylic acids and their water-soluble salts, such as their ammonium and alkali metal salts, quaternary salts of dimethylaminoethyl acrylate and dimethylaminoethyl methacrylate. Preferred nitrogen-bearing monomers within the above formula are acrylamide, dimethylaminoethyl acrylate quaternized with methyl chloride, dimethyl sulfate, or diethyl sulfate and dimethylaminoethyl methacrylate quaternized with methyl chloride, dimethyl sulfate, or diethyl sulfate. Preferred combinations of monomers include mixtures of acrylamide and sodium acrylate and mixtures of acrylamide and quaternary salts of dimethylaminoethyl acrylate. The invention also contemplates polymers obtained by polymerizing mixtures of the above defined monomers as well as water-soluble mixtures of such monomers with up to 10% by weight of other water-soluble, copolymerizable, ethylenically unsaturated monomers, for example, vinylimidazole, quaternary salts of vinylimidazole, acrylonitrile and maleic acid, and water-soluble salts of maleic acid, such as the alkali metal salts of maleic acid.

By selection of monomer combinations, polymers may be obtained which are nonionic, cationic, anionic, or ampholytic as desired. By way of illustration, acrylamide yields as essentially nonionic polymer, the quaternary salts of dimethylaminoethyl acrylate or dimethylaminoethyl methacrylate yield cationic polymers, copolymers of acrylamide with acrylic acid or with quaternized acrylic or methacrylic amine salts, are anionic or cationic, respectively, while terpolymers of acrylamide with both acrylic acid and the quaternary compounds are ampholytic.

Irradiation of the monomer is carried out in an aqueous solution containing about 10% to about 40%, and preferably about 10% to about 30% by weight of dissolved monomer. At the lower concentrations of monomer, the product is generally a pourable polymer solution; at concentrations of above about 15% by weight, the product is generally a nonpourable gel. The use of monomer concentrations below about 10% tends to be uneconomical and the use of monomer concentrations much above about 40% tends to restrict the amount of salt which can be dissolved in the monomer solution. Of course, the particular limits of monomer concentration vary somewhat with the particular monomers used and the radiation conditions used; but, in general, values within the ranges stated are satisfactory. It has been found that provided all other variables are kept constant, the intrinsic viscosity of the polymer product increases as the monomer concentration increases.

The intensity of radiation has an effect on the molecular weight of the polymer product obtained and consequently on the intrinsic viscosity of its solutions. In general, the higher molecular weight products are obtained at the lower intensities of radiation. That is, under otherwise identical conditions, the intrinsic viscosity of the polymer tends to increase as the intensity of the radiation used decreases. On the other hand, the degree of conversion which can be obtained before an undue degree of insolubilization takes place is greater at higher intensities of radiation. In view of these considerations, it is usually desirable to employ radiation intensities of at least 1,000 rads and preferably of at least 5,000 rads per hour. In order to obtain the significantly higher molecular weight polymers of this invention, values below about 200,000 rads per hour are desirable; and for producing polymers having molecular weights in the highest range, it is preferred that values below about 100,000 rads per hour be employed.

The total radiation dose may influence the water-solubility of the polymer, as it has been found that too high a radiation dose may render the resulting polymer water-insoluble. It has also been found that under otherwise identical conditions, the intrinsic viscosity of the polymer tends to decrease as the total dose increases. Accordingly, the total radiation dose to which the polymerization system is subjected must be carefully chosen. The particular radiation dose used will depend somewhat on the radiation intensity used, the monomer concentration, the particular monomer employed, the salt concentration, the particular salt used, and on the desired intrinsic viscosity of the polymer to be produced. Although a lower dose may be used, it is generally preferred to use a dose of at least 1,000 rads. The upper limit of radiation dose is that which produces substantial amount of water-insoluble products. However, for most practical purposes, dosages up to about 30,000 rads and preferably up to about 15,000 rads are employed.

Although the polymerization reaction can be stopped at any monomer to polymer conversion, the advantages of this invention are better realized by conversions from 80% to 100% and preferably from 85% to 98%. At these high monomer to polymer conversions the polymerization raw product is less vulnerable to cross-linking by post irradiation polymerization of unreacted monomer and therefore can be safely stored at room or lower temperatures or used as such if it is economically preferred. Partial polymerization of salt containing high monomer concentration solutions yields extremely high intrinsic viscosity polymers. However, the raw product of such polymerization has to be immediately processed to powder and the unreacted monomer disposed of or recycled.

The variables of radiation intensity, total radiation dose, salt concentration, and monomer concentration discussed above are interdependent variables. While useful polymers may be prepared at all monomer concentrations, salt concentrations, radiation intensities, and radiation dosages within the ranges given heretofore, all combinations of concentration, dose, and intensity within these ranges may not be used to prepare the water-soluble, high molecular weight polymers. In view of this interpendency of intensity, dose, salt concentration, and monomer concentration, it may be necessary to perform a limited amount of experimentation in order to prepare a polymer having the desired water solubility and intrinsic viscosity. However, this experimentation may be kept to a minimum in view of the disclosure in the numbered examples given hereinafter showing the preparation of a variety of polymers of different intrinsic viscosities and in view of the discussion herein of the effect of intensity, dose, salt concentration, and monomer concentration on the water-solubility and intrinsic viscosity of the polymer. For example, a polymer having an intrinsic viscosity of about 22 deciliters per gram in 2 normal sodium chloride solution at 25.5° C. may be prepared using the reaction conditions employed in Example 2 for the preparation of a polymer having an intrinsic viscosity of 24.2, except that the intensity is increased and/or the monomer or salt concentration is lower.

The radiation-induced polymerization reaction may be conducted over a rather wide range of pH. However, it is preferable to avoid the use of very low pH values because some production of undesirable, insoluble products tends to take place if the pH is unduly lowered, particularly with acrylamide base products. On the other hand, very high pH values may result in some hydrolysis and modification of the monomer being reacted, this again being particularly true of acrylamide. Although the particular range will depend to some degree on the particular monomer composition being polymerized and on the particular water-soluble salt used, it may be stated in general that pH values of about 2 to 12 are satisfactory. A preferred pH range for the preparation of anionic polymers is from about 8 to about 11.A preferred pH range for the preparation of cationic polymers is from about 3 to about 8.

The discussion of pH in the preceding paragraph applies primarily when the water-soluble salt used is a potassium, sodium, or lithium salt. When an ammonium salt is used, the pH of the reaction mixture is maintained from about 7 to about 9. When an aluminum salt is used, the reaction medium is maintained from about 2 to about 5to avoid precipitation of the salt.

The product of the process of this invention is an aqueous solution of the water-soluble polymer, which may be in the form of a pourable liquid or a nonpourable, rubbery gel, depending upon the polymer concentration and the intrinsic viscosity of the polymer. The bulk viscosity of the polymer solution tends to increase as the polymer concentration and intrinsic viscosity of the polymer increases. The polymer solutions produced, in accordance with this invention, may be admixed with water and used directly or the polymer solution may be concentrated by conventional means or it may be recovered in particulate form, for example, powder form. For example, a nonpourable gel may be finally subdivided and the water removed by conventional drying techniques or the water may be extracted from the subdivided gel with a water-miscible, volatile organic liquid which has no affinity for the copolymer, for example, methanol.

Anionic and nonionic polymers prepared by the process of this invention generally have an intrinsic viscosity of at least 20 deciliters per gram, and preferably at least 25 deciliters per gram. Cationic polymers prepared by the process of this invention generally have an intrinsic viscosity of at least 4 deciliters per gram, and preferably at least 14 deciliters per gram. Although polymers having a lower intrinsic viscosity may be prepared readily by the process of this invention, no advantage is gained by using this process to prepare such low molecular weight polymers. These low molecular weight polymers may be prepared easily in the absence of a water-soluble salt by known processes. Anionic and nonionic polymers having an intrinsic viscosity of 50 deciliters per gram, or higher, and cationic polymers having an intrinsic viscosity of 25 deciliters per gram, or higher, may be prepared by the process of this invention.

For purposes of this application, the intrinsic viscosity of nonionic polymers, cationic polymers, and polymers having an anionic content of below 85% are measured in two normal sodium chloride solution at 25.5° C., and the intrinsic viscosity of polymers having an anionic content of from 85% to 100% are measured in two normal sodium hydroxide solution at 25.5° C. When the intrinsic viscosity of the polymer is measured in water without the presence of salt or sodium hydroxide, values obtained are higher, particularly in the case of ionic polymers and polymers having an intrinsic viscosity of above 10.

Polymers prepared by the process of this invention have a Huggins constant of from 0 to about 0.8, and preferably from 0 to about 0.5. A preferred class of anionic and nonionic polymers have a Huggins constant from 0 to about 0.2. A preferred class of cationic polymer has a Huggins constant from about 0.1 to about 0.4.

The Huggins constant of a polymer is a measure of the degree of branching of the polymer. For two polymers having similar molecular weight but different Huggins constant, the lower Huggins constant indicates a more linear polymer. Briefly, the Huggins constant of a polymer, and thus the amount of branching, may be measured from a determination of the plot of the specific viscosity divided by concentration versus the concentration. The slope of this curve divided by the square of the intrinsic viscosity yields the Huggins constant. A more detailed definition of Huggins constant and method for determining Huggins constant of a polymer is found in "Textbook of Polymer Chemistry," Billmeyer, Interscience Publishers, New York, 1957, pages 125-139. For purposes of this application, the Huggins constant of polymers are determined by the process described by Billmeyer and using polymer concentrations below about 0.05 deciliters per gram.

The invention will be better understood from a consideration of the following examples which are presented for illustrative purposes and are not to be considered as defining or limiting the scope of this invention. All parts and percentages are by weight unless otherwise specified. The intrinsic viscosities reported are measured in 2 normal sodium chloride solution at 25.5° C., unless otherwise stated.

EXAMPLE 1

240 ml. of deionized water are added to a 500 ml. beaker. 0.6 gram of glacial acrylic acid, 30 grams of acrylamide, and 30 grams of sodium sulfate are dissolved in the water. The pH of the resulting solution is adjusted to 10.0 with aqueous sodium hydroxide solution. The solution is then added to an irradiation vessel and flushed with nitrogen for 20 minutes. The solution is irradiated with gamma rays from a cobalt 60 source for 16 minutes at a radiation intensity of 20,000 rads per hour. The reaction product is removed from the radiation source and allowed to stand until it has cooled to room temperature. The monomer to polymer conversion is 94.7%. A portion of the gel is extruded and a weighed amount of the gel strands are added to a beaker containing methanol. The polymer strands are allowed to stand in the methanol overnight. The strands are then ground on a Wiley mill using a 20 mesh screen. The ground product is slurried in methanol, filtered by vacuum filtration, washed three times on a filter with fresh methanol, and partially dried on the filter. The semi-dried powder is then dried in a vacuum oven for 24 hours at 36° C. The dried polymer powder is soluble in distilled water and has an intrinsic viscosity of 14.8 deciliters per gram and a Huggins constant of 0.27.

EXAMPLE 2

A solution containing 210 ml. deionized water, 0.9 gram glacial acrylic acid, 45 grams acrylamide, and 45 grams of sodium sulfate are prepared in accordance with the procedure of Example 1 and the pH of the solution adjusted to 9.6 with an aqueous solution of sodium hydroxide. The solution is placed in a reaction vessel, flushed with nitrogen for 20 minutes, and irradiated with gamma rays from a cobalt 60 source for 10 minutes at an intensity of 20,000 rads per hour. The product is then allowed to cool to room temperature. The polymerization product is a nonpourable, rubbery gel and the monomer to polymer conversion is 96.0%. Both the rubbery gel and polymer powder recovered from the gel are soluble in distilled water. The polymer has an intrinsic viscosity of 24.2 deciliters per gram and a Huggins constant of 0.10.

EXAMPLE 3

10 pounds, 10 ounces deionized water, 6.25 ounces of sodium hydroxide, 11 ounces of glacial acrylic acid, and 1 pound, 3.25 ounces of anhydrous sodium sulfate, and 2 pounds, 1.5 ounces of acrylamide are added to a 5 gallon polyethylene vessel in the order listed. The pH of the solution is adjusted to 9.5 with sodium hydroxide. The monomer solution is then irradiated for 16 minutes with gamma rays from a cobalt 60 source at an intensity of 20,000 rads per hour. The monomer to polymer conversion is 83.4%. The reaction product is a completely water-soluble rubbery gel. The resulting polymer is water soluble and has an intrinsic viscosity of 29.3 deciliters per fram and a Huggins constant of 0.15.

EXAMPLE 4

An aqueous solution containing 8 pounds, 3 ounces of dionized water, 8 ounces of sodium hydroxide, 14.5 ounces of glacial acrylic acid, 2 pounds, 12 ounces of acrylamide, and 1 pound, 3.25 ounces of anhydrous sodium sulfate are prepared and the pH of this solution adjusted to 9.5 with concentrated sodium hydroxide in accordance with the procedure of Example 3. The solution is then placed in a reaction vessel and flushed with nitrogen for 25 minutes. The solution is then irradiated for 6.5 minutes with gamma rays from a cobalt 60 source at a radiation intensity of 20,000 rads per hour. The resulting gel is removed from the gamma source and allowed to cool to room temperature. The monomer to polymer conversion is about 47% by weight, based on the weight of total monomer originally present in the solution. A portion of the gel is extruded and a weighed amount of the gel strands hardened in an excess of methanol. The hardened strands are ground on a Wiley mill using a 20 mesh screen. The ground product is slurried in methanol, suction filtered, and dried in a vacuum for 24 hours at 36° C. The resulting polymer is soluble in distilled water and has an intrinsic viscosity of 40.0 deciliters per gram and a Huggins constant of 0.0.

EXAMPLE 5

A solution containing 100 ml. of deionized water, 150 ml. of a 40% aqueous solution of the dimethyl sulfate quaternary salt of dimethyl amino ethyl acrylate, 60 grams of sodium chloride, and 20 grams of acrylamide is prepared and irradiated with gamma rays from a cobalt 60 source in accordance with the procedure of Example 1. The solution is irradiated for 14 minutes at an intensity of 20,000 rads per hour. The resulting product is a nonpourable, rubbery gel. The rubbery gel is then removed from the influence of gamma rays and cooled to room temperature. The monomer to polymer conversion is 83.2%. The polymer is soluble in water and has an intrinsic viscosity of 18.7 deciliters per gram and a Huggins constant of 0.06.

EXAMPLES 6-42

To a reaction vessel containing the indicated amount of deionized water are added the indicated amounts of monomer and salt. The pH is adjusted to the indicated value using 10 normal sodium hydroxide solution. The solution is flushed with nitrogen gas for 20 minutes and the vessel closed. The sample is irradiated with gamma radiation from cobalt 60 at an intensity of 20,000 rads per hour until the indicated total dose of radiation has been received by the sample. The sample is then removed from the radiation chamber for processing. The intrinsic viscosity and Huggins values are determined by standard techniques. The percent conversion of monomer to polymer is determined by weighing a portion of the reaction product, extracting product with methanol to precipitate the polymer, and drying the polymer in vacuum to constant weight. The weight of the dried polymer divided by the theoretical weight of product that would have been obtained if 100% conversion of monomer was achieved gives the percent conversion of monomer to polymer. The results are given in the following Table I.

                                      TABLE I                                      __________________________________________________________________________                                         Monomer                                                  Acrylic   (1)   (2)   Concen-                                    Ex.                                                                               Water  NaOH                                                                               Acid                                                                               Acrylamide                                                                           DMAEA-Q                                                                              DMAEM-Q                                                                              tration                                                                             NaCl                                  No.                                                                               ml.    gms.                                                                               gms.                                                                               gms.  gms.  gms.  (%)  gms.                                  __________________________________________________________________________      6 354    12.8                                                                               23.0                                                                               70.0              20                                          7 396    9.6 52.5                                                                               52.5              15                                          8 927    23.9                                                                               43.1                                                                               131.2             15                                          9 325    11.5                                                                               20.7                                                                               63.0              18   80.0                                  10 20,357 707.8                                                                              1245.8                                                                             4190.3            22                                         11 270    6.8 12.3                                                                               64.0              19.9                                       12 300    1.7 3.1 76.0              19.5                                       13 300            75.0              18.3                                       14 300            75.0              17.9 45.0                                  15 300            75.0              17.9                                       16 320            60.0              15.0                                       17 300            60.0              15.0                                       18 310            60.0              15                                         19 310            60.0              15   15.0                                  20 310            60.0              15   10.0                                  21 290            60.0              15   50.0                                  22 290            60.0              15                                         23 290            60.0              15   25.0                                  24 290            60.0              15                                         25 200            60.0              15   25.0                                  26 200            60.0              15   17.0                                  27 220                  80          25                                         28 210                  80          25                                         29 200                  80          25                                         30 215                  60          17.9                                       31 180                  80          25   60                                    32 28,468 495.5                                                                              906.0                                                                              6,653             18   6,965                                 33 28,256 1005.0                                                                             1798.0                                                                             5,479             18   6,951                                 34 621    42.2                                                                               75.8                                                                               121.0             22   140                                   35 318    25.7                                                                               46.3                                                                               49.5              22   60                                    36 260            60          20                                               37 255            60          20                                               38 290            60                     25                                    39 390            20    60               30                                    40 280            80                     (3)                                   41 311.4          49.5   16.5            (4)                                   42 247                  94               29.4                                  __________________________________________________________________________                   Salt                                                                           Concen-   Irradiaton  Intrinsic                                  Ex.                                                                               Na.sub.2 SO.sub.4                                                                     K.sub.2 HPO.sub.4                                                                  tration   Time  Conversion                                                                           Viscosity                                                                           Huggins                               No.                                                                               gms.   gms.                                                                               (%) pH    (minutes)                                                                            (%)   dl/gm                                                                               Constant                              __________________________________________________________________________      6 40.0       8   9.6   16    88.6  29.3 0.15                                   7 25.0       5   9.6   17    78.5  21.0 0.18                                   8 125.0      10  9.6   15    83.4  25.0 0.14                                   9            16  9.6   11    87.2  31.0 0.04                                  10 1075.9     6   9.6   16    84.8  30.6 0.14                                  11 50.0       12.4                                                                               9.6   12    90.6  28.7 0.07                                  12 30.0       7.3 9.6   12    93.5  28.3 0.07                                  13 35.0       8.5 10.0  12    92.5  25.0 0.10                                  14            10.7                                                                               10.0  13    100   23.5 0.12                                  15        45.0                                                                               10.7                                                                               10.0  10    91.3  26.2 0.12                                  16 20.0       5.0 10.0  13    90.8  19.0 0.21                                  17 40.0       10.0                                                                               10.0  13    93.7  21.0 0.16                                  18 30.0       7.50                                                                               10.0  12    95.5  19.7 0.17                                  19 15.0       7.50                                                                               10.0  12    86.7  22.1 0.14                                  20 10.0   10.0                                                                               7.50                                                                               10.0  12    90.0  20.9 0.13                                  21            12.50                                                                              10    10    94.0  21.5 0.12                                  22 50.0       12.50                                                                              10    10    93.0  22.3 0.09                                  23 25.0       12.50                                                                              10    10    90.0  24.3 0.12                                  24 25.0   25.0                                                                               12.50                                                                              10    10    89.4  26.0 0.08                                  25        25.0                                                                               12.50                                                                              10    10    88.1  23.3 0.10                                  26 16.0   16.0                                                                               12.50                                                                              10    10    88.2  24.6 0.09                                  27 20         6.25                                                                               3     40    100   3.4                                        28 30         9.4 3     40    100   3.8                                        29 40         12.5                                                                               3     30    100   4.8                                        30 60         17.9                                                                               3     30    98.2  4.2                                        31            18.75                                                                              3     40    90.3  4.8                                        32            16  9.5   11    89.4  33.0 0.05                                  33            16  9.5   11    82.5  32.5 0.08                                  34            14  9.5   12    84.0  32.3 0.08                                  35            12  9.5   13    84.0  30.8 0.08                                  36 24         6.0 3.0         87.3  12.3 0.36                                  37 48         12.0                                                                               3.0         90.3  11.8 0.34                                  38        25  12.5                                                                               10    10    88    25.4 0.1                                   39 30         15.3                                                                               3.0   12    63.0  11.8                                       40            10  9.0   12    91.6  24.4                                       41            5.2 3.0   14    84.4  16.8 0.26                                  42 29.4       14.7      20    72.5  4                                          __________________________________________________________________________      (1) Dimethyl sulfate quaternary salt of dimethylaminoethyl acrylate            (2) Dimethyl sulfate quaternary salt of dimethylaminoethyl methacrylate        (3) 40 grams of ammonium sulfate                                               (4) 20.6 grams of aluminum sulfate                                       

EXAMPLE 43

325 grams of deionized water are added to a 500 ml. beaker. 11.5 grams of sodium hydroxide, 20.7 grams of glacial acrylic acid, 63.0 grams of acrylamide, 20 grams of sodium sulfate, and 60 grams of sodium chloride are dissolved in the water. The pH of the resulting solution is adjusted to 9.5 with aqueous sodium hydroxide solution. The solution is then added to an irradiation vessel and flushed with nitrogen for 20 minutes. The solution is irradiated with gamma rays from a cobalt 60 source for 10 minutes at a radiation intensity of 20,000 rads per hour. The reaction product is removed from the radiation source and allowed to stand until it has cooled to room temperature. The resulting product is an aqueous gel of a water-soluble, ultra high molecular weight polymer. The polymer has a higher molecular weight than would be obtained under the same radiation conditions and a comparable amount of a single salt.

The water-soluble, high molecular weight polymers prepared by the process of this invention may be employed in any process where water-soluble, high molecular weight polymers are used. Polymers produced by the process of this invention are particularly suitable for use as flocculating agents, viscosity control agents in aqueous solutions and as mobility control agents in flood water for the recovery of crude oil from subterranean reservoirs.

Although the process of this invention has been described with reference to specific reaction conditions and reactants, it will be apparent that still other different and equivalent reactants and process conditions may be substituted for those specifically described, all within the spirit and scope of this invention. 

Having described the invention, what is desired to be secured by Letters Patent is:
 1. A process for preparing a water-soluble, substantially linear, high molecular weight polymer which comprises irradiating an aqueous solution having a pH from about 2 to about 12 and containing from about 10% to about 40% by weight of: (a) an ethylenically unsaturated monomer having the formula (I) ##STR3## (b) mixtures of monomers of formula (I), or (c) mixtures of at least one of the monomers of formula (I) with up to 50% by weight of an ethylenically unsaturated monomer selected from the group consisting of vinyl sulfonic acid, alkali metal salts of vinyl sulfonic acid, diacetone acrylamide, and mixtures thereof, provided that said mixture of monomers is water-soluble, wherein R represents hydrogen or methyl and Y represents --NH₂, --OM, ##STR4## wherein M is hydrogen H₄ N⁺, alkali metal, or any other cation yielding a water-soluble, polymerizable compound, R₁, R₂, and R₃ are 1 to 4 carbon alkyl radicals and X is an anion, and containing at least about 3% by weight of a salt selected from the group consisting of potassium sulfate, potassium chloride, potassium fluoride, potassium bisulfate, tribasic potassium phosphate, dibasic potassium phosphate, monobasic potassium phosphate, sodium sulfate, sodium bisulfate, sodium chloride, tribasic sodium phosphate, dibasic sodium phosphate, monobasic sodium phosphate, lithium sulfate, lithium bisulfate, lithium chloride, ammonium sulfate, ammonium bisulfate, ammonium chloride, tribasic ammonium phosphate, dibasic ammonium phosphate, monobasic ammonium phosphate, ammonium fluoride, aluminum sulfate, aluminum chloride, and mixtures thereof, with high-energy ionizing radiation at an intensity of from 1,000 to 200,000 rads per hour to a total radiation dose of from about 1,000 to about 30,000 rads, to form an aqueous solution of a water-soluble, substantially linear, high molecular weight polymer, provided that when the aqueous solution contains at least 3% by weight of a water-soluble salt selected from the group consisting of aluminum sulfate, aluminum chloride, and mixtures thereof, the unsaturated monomer consists of from 5% to 100% of cationic monomer and from 95% to 0% nonionic monomer, and the aqueous solution has a pH from about 2 to about 5, and provided that the aqueous solution has a pH of from about 7 to about 9 when the water-soluble salt is an ammonium salt.
 2. A process of claim 1 wherein the radiation intensity is from about 5,000 to about 200,000 rads per hour and the radiation dose is from about 1,000 to about 30,000 rads.
 3. A process of claim 1 wherein the radiation intensity is from about 5,000 to about 100,000 rads per hour and the radiation dose is from about 1,000 to about 15,000 rads.
 4. A process of claim 2 wherein the monomer concentration is from about 10% to about 30% by weight, based on the total weight of solution.
 5. A process of claim 2 wherein the aqueous solution contains at least one monomer selected from the group consisting of acrylamide and methacrylamide and at least one monomer selected from the group consisting of alkali metal and ammonium salts of acrylic acid and alkali metal and ammonium salts of methacrylic acid.
 6. A process of claim 5 wherein the monomer concentration is from about 10% to about 30% by weight, based on the total weight of solution.
 7. A process of claim 6 wherein the monomer is a mixture of from about 50 to about 99 weight percent of acrylamide and from about 1% to about 50% by weight of at least one monomer selected from the group consisting of acrylic acid, and alkali metal salts of acrylic acid.
 8. A process of claim 7 wherein the monomer mixture contains from about 50% to about 80% by weight of acrylamide and from about 50% to about 20% by weight of sodium acrylate.
 9. A process of claim 1 wherein a mixture of salt is used.
 10. A process of claim 1 wherein the salt is sodium chloride.
 11. A process of claim 1 wherein the salt is sodium sulfate.
 12. A process of claim 1 wherein the salt is a mixture of sodium chloride, sodium sulfate, and potassium dibasic phosphate.
 13. A process of claim 1 wherein the salt is a mixture of sodium chloride and sodium sulfate.
 14. A process of claim 1 wherein the salt is a mixture of sodium chloride and potassium dibasic phosphate.
 15. A process of claim 1 wherein the salt is a mixture of sodium sulfate and potassium dibasic phosphate.
 16. A process of claim 1 wherein the percent of conversion of monomer to polymer is from 80% to 100% by weight.
 17. A process of claim 1 wherein the salt concentration is from about 4% by weight, based on the total weight of solution to saturation at 35° C.
 18. A water-soluble, substantially linear, high molecular weight polymer prepared by a process of claim
 1. 19. A process of claim 1 wherein the salt is selected from the group consisting of sodium chloride, sodium sulfate, sodium bisulfate, potassium chloride, potassium sulfate, potassium bisulfate, ammonium chloride, ammonium sulfate, ammonium bisulfate, and mixtures thereof.
 20. A process of claim 1 in which the aqueous solution contains at least one monomer selected from the group consisting of acrylamide and methacrylamide and at least one monomer selected from the group consisting of quaternary salts of dimethylaminoethyl acrylate and dimethylaminoethyl methacrylate. 